1. Field of the Invention
Implementations of various technologies described herein generally relate to interpreting data obtained with well logging tools.
2. Description of the Related Art
The following descriptions and examples are not admitted to be prior art by virtue of their inclusion within this section.
Determining the porosity and fluid content of subsurface earth formations are critical elements in maximizing the efficiency of oil, gas, and water (“formation fluids”) exploration. In order to improve formation fluids exploration, drilling, and production operations, it may be necessary to gather as much information as possible on the properties of the underground earth formations as well as the environment in which drilling takes place.
The collection of downhole information, also referred to as logging, may be realized in different ways. A well tool, having sources and sensors for measuring various parameters, may be lowered into the borehole on the end of a cable, or wireline. The cable, which may be attached to a mobile processing center at the surface, is the means by which parameter data is sent up to the surface. With this type of “wireline” logging, it becomes possible to measure borehole and formation parameters as a function of depth, i.e., while the tool is conveyed along the wellbore.
An improvement over wireline logging techniques is the collection of data on downhold conditions during the drilling process. By collecting and processing such information during the drilling process, the driller can modify or correct key steps of the operation to optimize performance and avoid financial injury due to well damage, such as collapse or fluid loss. Formation information collected during drilling also tends to be less affected by the drilling fluid (“drilling mud”) invasion processes or other undesirable influences as a result of borehole penetration, and therefore are closer to the properties of the virgin formation.
Schemes for collecting data of downhole conditions and movement of the drilling assembly during the drilling operation may be known as measurement-while-drilling (MWD) techniques. Similar techniques focusing more on measurement of formation parameters than on movement of the drilling assembly may be known as logging-while-drilling (LWD). However, the terms MWD and LWD are often used interchangeably, and use of either term herein includes both the collection of formation and borehole information, as well as data on movement of the drilling assembly. Other logging operations may be carried out using coiled tubing, slick lines, logging-while-tripping, and permanent monitoring applications as known in the industry.
MWD or LWD logging tools can provide an extensive set of measurements that contain valuable information on underground earth formations and environmental parameters. In most cases, interpretation of the information is necessary to extract the useful information. The raw data may be affected to varying degrees by effects such as borehole effect, tool eccentricity, invasion, anisotropy, shoulder-bed effects, and the like.
A shoulder bed refers to a formation layer above or below the layer being measured by a logging tool. The term is used in particular in resistivity logging to describe the layers above and below a formation. Some resistivity tools, such as induction and propagation tools, can sense beds located tens of feet from the measure point and can be significantly affected by shoulder beds, even when the reservoir is thick.